Downgrading Lion...

I seem to have a problem I thought would be easy to fix, but seems to have escalated to a more complex one. I have an iMac (actually my dad's) which was recently upgraded from Snow Leopard to Lion. Of course he had that one program that wasn't compatible, so he asked me to downgrade it. There is NO Time Machine (or any other for that matter) backup of the system.

I thought, "This will be easy. I will just make a new partition, boot from the SL disc that came with the computer, install on the new partition, select the option to restore from a previous Mac, and restore from the other partition."

I can do this on my (older) laptop no problem. However, the iMac won't startup from the Snow Leopard disc. All the hardware and the disc is fine. I believe the reason is the SL disc is OS X revision 3 (10.6.3) and I think the computer actually came with 10.6.4. (He bought the iMac from some retail store and 10.6.4 had just recently came out. I think the guy upgraded it but left the 10.6.3 CD in there??)

So, (this part may seem silly) my question is, is there any way of possibly upgrading a USB version of the boot disk? I can copy the disc to a USB drive and get it to boot on my laptop, but is there a way of making the installer 10.6.4 (or any newer version) somehow?

If that won't work well, is there any other way of getting SL on the machine? I have the collective updates for it, so even if I can force install the older version, I can upgrade it no problem. (Maybe I've convinced my dad to keep backups now)

Does your Mac notebook have Firewire?
If so, use TargetDiskMode and boot the iMac into TDM and connect it to your Mac and then backup all the relevant data from it to another HDD.
Then use the Mac OS X 10.6 Snow Leopard DVD you have and install 10.6 onto the HDD of the iMac (after it has been properly formatted*) and run all the updates and then boot the iMac normally and see, if that works, as it should work.

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